The American Community Survey (ACS) and the Puerto Rico Community Survey (PRCS) ask respondents age 1 year and over whether they lived in the same residence 1 year ago. For people who lived in a different residence, the location of their previous residence is collected.

ACS uses a series of monthly samples to produce estimates. The 5-year dataset is used for the county-to-county migration flows since many counties have a population less than 20,000.